FBSession.activeSession.isOpen gives NO even i logged in previously in iOS SDK

Question

I am using the Facebook Graph API in my iOS application to post a message to Facebook. I am able to post the message, but I am facing an issue. When I launch the app it asks Facebook credentials (it display the login page again even I logged in successfully before). I am unable to find what I am missing.

if (FBSession.activeSession.isOpen)
{

    [FBRequestConnection startForPostStatusUpdate:@"Sample Text" completionHandler:^(FBRequestConnection * connection , id result , NSError * error){
        if (!error)
        {
            NSLog(@"Posted success fully");
        }
        else
        {
            NSLog(@"Error %@",error);
        }
    }];
}
else
{
    NSArray *permissions = [NSArray arrayWithObjects:@"publish_actions", @"publish_stream", nil];

    NSLog(@"Need to login");
    [FBSession openActiveSessionWithPublishPermissions:permissions defaultAudience:FBSessionDefaultAudienceFriends allowLoginUI:YES
                                                             completionHandler:^(FBSession *session, FBSessionState state, NSError *error) {


                                                                    [FBSession setActiveSession:session];
                                                                 NSLog(@"Susccess %@",session.accessTokenData);
                                                             }];